The present invention is directed to the preparation of a class of interpolymer membranes based on the hydrophilic polyamideimide of [1,.omega.-alkylene(2,6-diketopiperazine)1,4-dicarboxamide] (hereafter sometimes referred to as ADD). The membranes (by empolying modifications of composition and procedure) are applicable as selective membranes for dialysis and solvent extraction processes, as membranes in reverse osmosis processes, as ultrafiltration membranes in pressure-drive processes and in hollow fibers structures.
The conventional solvent-type reverse osmosis membranes require a substantial hydrostatic pressure be applied to the solution to be purified on the side of the membranes that contains the solvent or the desired component through the membrane leaving the undesirable component or solute behind because the membrane is relatively impermeable to the undesirable solute. Reverse osmosis processes require very high pressures, for examples, pressures of about 600 to 1,000 psi above the reversible osmotic pressures are commonly employed. Additionally, reverse osmosis membranes have comparatively limited applications because they are relatively nonselective and customarily permeable only to the solvent, e.g., water. Accordingly, reverse osmosis membranes are not ordinarily especially useful in separating the dissolved components of a solution as is often required in the treatment of mixtures. Further, when used to treat solutions containing high molecular weight material such as proteins or dissolved organic matter, they tend to get clogged or readily poisoned as the organic matter is selectively absorbed by the membrane. Organic matter can either coat the surface or make it hydrophobic and thereby prevent the transport of water across it. Specifically, as in desalination and other water renovation processes, the organic matter can penetrate the membrane, disrupt the hydrogen-bonded water structure, which is apparently responsible for the selective action of the membranes, and thereby destroy either the selective action of the membrane or its high flux rate, or both.
Ultrafiltration membranes capable of separating dissolved solutes of molecular weights ranging from those of the common salts to proteins have been made in the past by mixtures of polyelectrolytes held together by purely ionic bonds as complexes of polyacids and polybases, deposited upon a hydrophobic, porous matrix, or they are porous films of largely hydrophobic polymers usually formed by coagulation techniques. These polymer complexes have the disadvantages of chemically not being crosslinked, and consequently are relatively unstable and more susceptible to fouling.
The purpose of the present invention is directed to novel formulations and methods for preparing membranes which are useful for a variety of applications. For example, when used as ultrafiltration media, the membranes of the present invention may be used to separate dissolved materials using a variant of the more familiar reverse osmosis process. Additionally, the membranes of the present invention may be used in separation processes involving passive diffusion of materials across the membrane by virtue of concentration gradients in passive dialysis. Moreover, these membranes are especially useful for 2-phase solvent-extraction processes.
As contrasted with cast membranes of ionic character, as described in the prior art, the novel membranes of the invention are chemically crosslinked by covalent bonds to the degree heretofore not attainable, so they retain their particularly desirable properties for prolonged periods of time, such corsslinking having been achieved from a membrane cast from a solution whose homogeneity was maintained over a wide range of compositions.